删除所有迭代中的尾随逗号

Question

我知道如何删除尾随逗号，但我似乎无法弄清楚如何从所有迭代中删除尾随逗号，因为我的代码只删除最后一个。我得到的输出是调用此方法时，以下多次：

A:on 0 -> B, on 1 -> A, 
B:on 0 -> C, on 1 -> A, 
C:on 0 -> C, on 1 -> A 

---

string FSA::transitions_to_string(string s) { 

    std::ostringstream oss; 
    string output = ""; 
    map<string, string>::iterator itr1; 
    map<string, map<string, string> >::iterator itr2; 
    map<string, map<string, string> >::iterator itr3; 

    itr3 = table_.find(s); 

    if (itr3 == table_.end()) { 
     throw domain_error (s + " doesn't exist"); 
    } 

    for (itr2 = table_.begin(); itr2 != table_.end(); itr2++) { 
    oss << itr2->first << ":"; 
     for (itr1 = itr2->second.begin(); itr1 != itr2->second.end(); itr1++) { 
      oss << "on " << itr1->first << " -> " << itr1->second << ", "; 
      output = oss.str(); 
     } 

    oss << endl; 
    } 

    return output.substr(0, output.size() - 2); 
} 

Answer 1

而不是删除尾随逗号，如@BoundaryImposition笔记，你可以不会写

for (itr2 = table_.begin(); itr2 != table_.end(); itr2++) { 
    string delim = ""; // set blank for first element of each inner loop 
    oss << itr2->first << ":"; 
    for (itr1 = itr2->second.begin(); itr1 != itr2->second.end(); itr1++) { 
     oss << delim << "on " << itr1->first << " -> " << itr1->second; 
     delim = ", "; // after first loop this is a nop 
    } 
    oss << endl; 
} 
return oss.str();